1. Field of the Invention
The present invention relates to an optical nanofiber resonator adapted to generate photons and emit the generated photons.
2. Description of the Related Art
Recently, with increase in information communication traffic, there is a desire to develop a new technology capable of dramatically promoting high-level security guarantee technology and high-speed communication technology. As a communication technology capable of solving the problems, quantum information communication technology is being actively studied and developed around the world.
The quantum information communication technology is a communication technology for generating photons (quanta) and manipulating the generated photons, and is a new communication technology to which quantum mechanics principle is directly applied. To be specific, the quantum information communication technology is a technology for performing information communication with the photons (quanta) as a mediator, wherein the photons are transmitted through a transmission path such as an optical fiber while the photons are maintained in quantum state. In the quantum information communication, since information communication is performed with the photons as a mediator, single photons or correlated photon pairs (photons in pairs) are fundamental elements of information transmission.
One of the main objects of developing the quantum information communication technology is to achieve quantum cryptographic communication. Conventionally, methods for encrypting information are employed in various developed communication systems in order to prevent information interception during communication, but the encryption employed now is not always safe. However, this problem can be solved by the quantum cryptographic communication.
As described above, in the quantum information communication, communication is performed while the photons are maintained in quantum state. However, during transmission of the photons, if the information is viewed (i.e., intercepted) by a third person, the quantum state of the photons will change, and therefore the recipient will know that the information has been viewed by a third person. Thus, in the quantum cryptographic communication, if encryption key is sent using this nature of the quantum information communication, interception is impossible in principle.
Photon generation technology is an indispensable element technology for developing the aforesaid quantum information communication and quantum cryptographic communication. As a single photon generating device, there are conventionally proposed, for example, a device in which an InAs single quantum dot is arranged in an optical micropillar resonator produced by precisely nano-processing a GaAs semiconductor, and a device in which a single quantum dot is arranged in a silver nanowire. In these devices, the quantum dots are laser-excited to generate photons, and the generated single photons are emitted to a predetermined direction. In such single photon generating devices, photon generation efficiency is relatively high; however the loss while the emitted photons is coupled to the guided mode of the optical fiber (i.e., the loss while the emitted photons is introduced into the optical fiber) is large.
Further, as a single photon generating device, there is conventionally proposed a device using a micro-resonator configured by micro concave mirrors. In such a device, an atom is disposed between the micro concave mirrors, and the atom is laser-excited; and at this time, a strong coupled state is established between the light-emitting atom and the resonator, so that photons are emitted.
Further, in nonpatent documents such as Physical Review A, Vol. 79, 021801, 2009, there is proposed a single photon generating element using an optical fiber (referred to as “optical nanofiber” hereinafter) having a diameter of substantially half wavelength of the propagation light. FIG. 15 is a view schematically showing the configuration of a single photon generating element using an optical nanofiber.
A single photon generating element 100 includes an optical nanofiber 101, and single light emitters 102. The optical nanofiber 101 is produced by heating and elongating a part of a conventional optical fiber for communication used in a communication system to form an ultrafine portion 103, wherein the diameter of the optical fiber for communication is substantially in a range of, for example, several tens μm to 100 μm, and wherein the diameter of the ultrafine portion 103 is of nanometer order. Thus, the appearance of the optical nanofiber 101 is such that the diameter of the optical nanofiber 101 becomes continuously smaller along the extending direction from one end thereof, becomes a size of nanometer order near the center thereof (i.e., in the ultrafine portion 103), and then becomes continuously larger along the extending direction toward the other end thereof.
Two optical fiber portions 104 respectively formed at both ends of the optical nanofiber 101 have the same configuration as that of the optical fiber for communication, and include a core 105 and a clad 106. Further, the single light emitters 102 are disposed on the ultrafine portion 103.
Incidentally, in order to clearly show the configuration of the single photon generating element 100, the single light emitters 102 are shown in the same size as that of the ultrafine portion 103 of the optical nanofiber 101 in FIG. 15, but actually the diameter of the ultrafine portion 103 is about 400 nm while the diameter of the single light emitters 102 is about 5 to 10 nm. Further, in order to clearly show the configuration of the optical fiber portion 104, the ratio of the diameter of core 105 to the diameter of the clad 106 is enlarged in FIG. 15, but the actual ratio of the diameter of core 105 to the diameter of the clad 106 is about 1:10.
In the single photon generating element 100 shown in FIG. 15, the single light emitters 102 are irradiated with a laser beam 107 so as to be excited. Thus, a light (photons) having a predetermined wavelength is emitted by spontaneous emission from the single light emitters 102, and the emitted photons are channeled from the ultrafine portion 103 into the optical nanofiber 101. Further, the photons channeled into the optical nanofiber 101 are emitted to the outside through the optical fiber portions 104 that has the same configuration as that of a conventional optical fiber for communication.
FIG. 16 is a graph showing a measured result of the photons emitted from the optical fiber portion 104 of the single photon generating element 100 having the aforesaid configuration, the measured result being obtained by detecting the photons with a highly-sensitive photon detector through an optical fiber network. The horizontal axis of the characteristic graph of FIG. 16 represents measurement time, and the vertical axis represents output (counts) of the photon detector. Incidentally, such experiment was performed while continuously exciting the single light emitters 102 (i.e., the quantum dots).
It can be known from the measured result shown in FIG. 16 that, in the output characteristics of the photon detector, durations while the output value is close to zero are appeared from time to time, which means there are durations while no photon is detected. Since the single light emitters 102 are continuously excited as described above, if a plurality of photons are generated at the same time, several of the plurality of photons will inevitably be detected by the photon detector. In such a case, durations while no photon is detected will not appear in the output characteristics detected by the photon detector. In other words, the measured result shown in FIG. 16 indicates that it is possible to emit the single photon with the single photon generating element 100 that uses the optical nanofiber, and it is possible obtain the single photon through the optical fiber network.
Further, in the single photon generating element 100 that uses the optical nanofiber, since it is possible to directly channel the photons into an external optical fiber through the guided mode of the optical fiber, the problem of the loss while the photons emitted from a device using an optical micropillar resonator or a silver nanowire is introduced into the optical fiber can be solved.
However, in the method using the optical nanofiber, coupling efficiency between the light emitted by spontaneous emission from the single light emitters 102 and propagation mode of the optical nanofiber 101 is low (i.e., channeling efficiency of the photons from the single light emitters 102 into the optical nanofiber 101 is low), and that is a problem.